Mass spectrometer beam monitor

ABSTRACT

The ion beam from a field desorption source in a double focusing magnetic mass spectrometer is monitored by disabling the electric sector of the mass analyzer such that the ion beam is not deflected. An opening is provided in the wall of the electric sector such that the undeflected ion beam may pass therethrough to a detector. This permits the characteristics of the field desorption source to be ascertained more quickly and easily so that a mass analysis may be performed. The monitor may be operated automatically to vary a characteristic of the field desorption source until ions are detected. Thereafter, the electric sector is energized and an analysis performed.

BACKGROUND OF THE INVENTION

This invention relates to mass spectrometers and, more particularly, toan ion beam monitor that facilitates the use of mass spectrometers.

Mass spectrometers are well known for their use in analyzing unknownsamples by observing their mass spectra. To observe such mass spectrathe unknown sample is first converted into an ion beam which is massanalyzed in a well-known manner. Various high energy and low energysources are used to provide ions of the unknown sample.

In contrast to electron impact mass spectrometry (a high energy source),field desorption sources produce relatively uncomplicated mass spectrathat characterize the molecular weight of various materials. Thetechnique known as field desorption mass spectrometry has come intoextensive use in the last few years, particularly for the analysis oforganic compounds. Field desorption mass spectrometry utilizes stablefield desorption emitters having long dendrites capable of adsorbingsufficient sample to provide useful field desorption spectra. Such fielddesorption emitters are described by H. D. Beckey et al., J. Physics E.;Scientific Instruments, 6, 1043 (1973).

A field desorption ion source of conventional design produces positiveions of the sample applied to the emitter. Such ions are produced whenthe emitter is heated in an electric field of sufficient strength,usually 10⁷ volts/centimeter, to remove an electron from the samplemolecule, Such removal normally occurs at one of the many tips of thedendrites on the emitter. These ions are produced from the sample thatis applied to the emitter when and if two conditions are simultaneouslyachieved. The first is that the sample remains on the emitter as theemitter is heated. Secondly, proper for ionization of the sample mustexist within the temperature and electric field characteristics of thesource.

In the analysis of unknown materials, neither of these conditions areknown. When these uncertainties are added to the fact that the ions tobe expected in the analysis are not known and the operationaldifficulties associated with field desorption analyses, it is imperativethat the operator know when ions are being produced from the sample,irrespective of mass analysis. It would be highly desirable if one wereable to first learn the field desorption characteristics of the sampleand then perform the mass analysis. This would result in a greatreduction of the time required.

SUMMARY OF THE INVENTION

Accordingly it is an object of this invention to provide an improvedapparatus for determining the field desorption characteristics of asample.

Another object of this invention is to provide an improved system foreffecting field desorption analyses of samples.

A conventional ion beam analyzer includes a sample ion source forgenerating ions of a sample to be analyzed, means for extracting thesample ions from the source, means for focusing the extracted sampleions into a beam, separation means positioned along the ion beam forselectively deflecting species of ions, and detecting means fordetecting the selected species ions.

According to this invention, disabling means are added to the beamanalyzer for disabling at least a portion of the separation means suchthat the ion beam from the ion source remains undeflected. Sensing meansare located along the undeflected ion beam for sensing the sample ionswhen they do occur, and, finally, enabling means are coupled to thedisabling means for reenabling the mass separation means. This permitsthe operator to vary such features as source (emitter) position,temperature and electric field strength until ions are produced from theunknown sample. This permits a ready determination of the fielddesorption characteristics of the sample, i.e., when the sample isproducing ions. Once these characteristics are acquired, the operatormay readily reproduce such characteristics or select thosecharacteristics which are deemed most desirable for the particularanalysis to be performed.

The various emitter characteristics may be varied automatically ormanually; for example, the emitter current (and hence emittertemperature) may respond to the sensing means for automaticallyreenabling the mass separation means when the sample ions reach apredetermined intensity level. Automatic means may be used to vary thefield desorption characteristics until ions are produced. At this point,a mass analysis is performed following which the field desorptioncharacteristics may be further varied. One of the most easily automatedof these field desorption characteristics is that of emittertemperature.

DESCRIPTION OF THE DRAWINGS

Further advantages and features of this invention will become apparentupon consideration of the following description wherein:

FIG. 1 is a part diagrammatic and part block representation of anautomated analyzer system contructed in accordance with a preferredembodiment of this invention;

FIG. 2 is a part diagrammatic and part block representation of the massanalyzer of FIG. 1 depicting a field desorption emitter and a particularplacement of a detector for the ion beam; and

FIG. 3 is a timing diagram of emitter heating current, beam monitoroutput, electric sector voltage and magnetic sector current for aparticular operative embodiment of a system utilizing this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The overall system of this invention is depicted in the representationof FIG. 1. While this invention may find use with a mass analyzer usingany low energy ion source such as chemical ionization or photoionization, it will be described in conjunction with the preferred usagewhich is with a field desorption source. Field desorption sources areknown and are described, for example, in the said Beckey article. Such asource is depicted in FIG. 1 by the block 10. This field desorptionsource includes an emitter 12 (FIG. 2) as will be described hereinafter.This emitter 12 has an emitter heating current supply 14 which may becontrolled manually or, in a preferred embodiment, by a ramp generator16. The ramp generator may be any well-known generator capable ofgenerating an increasing current as a function of time such as providedby a power supply whose output is controlled by the charging of acapacitor. Function generators of this type are described, for example,in Chapter 7 of "IC OP-AMP Cookbook" by Walter G. Jung, copyright 1974by Howard W. Sams & Co., Inc., Indianapolis, Indiana. The ramp generatormay be energized by a manual switch 18 connected to a source ofpotential depicted by the battery 20. The ramp function generated by thegenerator 16 may be temporarily terminated or delayed, as will bedescribed in conjunction with FIG. 3, by an output signal, whichdisables the generator, from a one-shot multivibrator 22 ofpredetermined time duration as determined by the output characteristicof the one shot. The one-shot multivibrator 22 may be any conventionalcircuit.

Ions, generated by the ion source, are depicted by the dashed line 24 aspassing through a separation means 26 which, in the preferredembodiment, includes an electric sector 28 and a magnetic sector 30,both of wellknown design. An instrument incorporating such features,including the ion source 10 and an electron multiplier type detector 32at the output of the magnetic sector 30 is available from the E. I. duPont de Nemours and Company, Wilmington, Delaware. Such instrument issold as a Model 21-492B. The ions of beam 24 are deflected in theelectric sector 28 by an electrostatic field therein established by anelectric potential derived from an appropriate source depicted by theblock 34. In like manner the magnetic sector 30 is controlled by amagnetic sector power supply depicted by the block 36.

As is known, the ions leave the source 10 and are deflected in theelectric sector by the electrostatic field therein and then by themagnetic field of magnetic sector according to their respective mass tocharge ratios. The separated ions, thus separated by the separationmeans 26, are detected by the electron multiplier detector 32.

In accordance with this invention, an opening or a hole 38 is providedin the outside of one of the walls or field plates 44 of the electricsector 28, as will be described hereinafter in conjunction with FIG. 2,so that an undeflected beam of ions 40 may pass to an electronmultiplier beam monitor detector 42. To permit this undeflected path ofions to occur, the field plates 44 of the electric sector 28 are shortedtogether such that no deflecting field exist. Under these conditions theions proceed along a straight line path as depicted by the dashed line40. The ions thus leave the electric sector 28 and pass to the beammonitor 42.

The electron multiplier beam monitor 42 consists of a secondary electronmultiplier (SEM), being any one of several commercially available types.The anode (not shown) of the beam monitor is connected to the input of asolid state amplifier. In the preferred embodiment, the beam monitor 42is identical with the electron multiplier detector 32. As is well knownto those experienced in the practice of mass spectrometry, thesensitivity and most particularly the signal to noise ratio of thesecondary electron multiplier plus solid state amplifier is superior tothat of a conventional electrometer amplifier. Mass spectrometerspreviously used for field desorption analysis, such as described byBeckey hereinabove mentioned or many of those commercially available,have been limited in their ability to perform field desorption analysesdue to the low sensitivity and high noise level of an electrometer typebeam monitor. Such prior art beam monitors have typically beenpositioned adjacent the ion source. Electron multpliers cannot be solocated.

An electron multiplier is particularly advantageous in this applicationdue to the very low intensity of ions produced by the field desorptionion source 10. As has been reported by Beckey, most organic samples thatare analyzed by the field desorption technique are typically veryinvolatile and subject to thermal decomposition. Both of thesecharacteristics result in low intensity ion beams (typically 10⁻¹⁸ to10⁻¹⁴ amperes) being produced. A secondary electron multiplier detectorcan easily detect such low intensity signals whereas an electrometerdetector cannot.

The output of the beam monitor 42 is connected to a conventionaldetector, which in this one embodiment, is depicted as a conventionalchart recorder 46. This recorder may have either an electronicmicroswitch or photo beam detector for sensing the pen position suchthat when a predetermined, selectable amplitude of the ion beam 40 isdetected by the beam monitor 42, an output signal may be generated online 48. This output signal is connected to trigger the one-shotmultivibrator 22 and also is connected though a time delay network 50 tothe magnetic sector scan control 36. The output signal is also connecteddirectly to the electric sector on-off control 34.

While it is to be noted that the system may be operated with manualcontrols, including that of the ramp generator 16 (i.e., a potentiometermay be adjusted to vary the heater current), the automatic systemdepicted in FIG. 1 is preferred.

Thus in a typical operation an unknown sample to be analyzing using afield desorption ion source is placed upon the emitter of the source 10in a conventional manner. Next, the ramp generator 16 is turned on byclosing the switch 18. This causes the emitter heating current, asdepicted in the timing waveform of FIG. 3, to increase (in this case,linearly) as a function of time. The electric sector and magnetic sectorscan circuits 34 and 36, respectively, are off; i.e., plates 44 of theelectric sector 28 are shorted together such that a zero voltagedifferential is applied thereacross and there is no electric field tocause deflection of the ion beam 24. Similarly, the current supplied tothe magnetic sector deflection coils is constant, i.e., no scansiontakes place.

Under these conditions any ions produced in the ion source 10irrespective of energy and mass are all directed by the acceleratingpotential in the source along the straight line path 40 to the beammonitor 42. When a particular emitter temperature, due to the emitterheating current, is achieved (a field desorption characteristic of thesample), it will produce ions from the particular sample underinvestigation. These ions are detected by the beam monitor 42 producinga typical output signal as depicted by the waveform 52. When this signalreaches a predetermined level, the level is sensed by the sensor in therecorder 46. The sensor provides a trigger signal to the one-shotmultivibrator 22 whose output activates the electric sector supply 34,temporarily discontinues the ramp so that a momentary hold is placed onthe emitter heating current for the period of the one-shot pulse, andactivates a scansion by the magnetic sector scan 36 after a slight delayprovided by the delay 30. The ion beam 24 is deflected along the curvedpath 54 by the electric sector. A short time later, after anyinstability of the system has had a chance to stabilize, the magneticsector supply 36 effects a scansion, as depicted in FIG. 3 by themagnetic sector current waveform, to complete the deflection of the ionsto be detected by the detector 32 of the mass analyzer. Once theone-shot multivibrator pulse is terminated, both the electric sector andmagnetic are returned to their "off" condition and the emitter heatingcurrent allowed to continue its rise. Perhaps another temperature willbe reached at which ions occur, perhaps not; it depends on the fielddesorption characteristics of the sample. The pulse from the one-shot22, is of sufficient duration to permit a complete scansion of themagnetic sector.

Other field desorption characteristics of the sample include emitterposition and electric field within the ion source. These may also bevaried either manually or automatically. For example, the electric fieldmay be varied by known means, such as by a potentiometer, or byvariation of the voltage of the various supplies depicted in FIG. 2. Inthis latter event, the one-shot multivibrator instead of being connectedto the ramp generator for the emitter heating current supply, will beconnected to a similar ramp generator (not shown) for a voltagecontrolled power supply such as the positive potential supply 60 or thenegative potential supply 62.

In conventional field desorption apparatus, some samples fail to beionized. The system of this invention will permit this determination inone or two loadings of a sample. In contrast the field desorptionsources of the prior art require many loadings and even then one cannotalways be certain whether ions are produced or not. If a manual systemis used, the recorder will still be preferably used so that thecharacteristic point at which ions occur will be recorded for futurereference. Alternate automatic modes of operation are also possible; forexample, heater current and field strength in the source may be variedsimultaneously.

Some of the elements of the system illustrated in FIG. 1 are shown ingreater detail in FIG. 2. Thus the ion source 10 is shown to include afield desorption emitter 12 of conventional design connected to theemitter heating current supply 14. A positive potential supply 60 isconnected to the emitter 12. Accelerating electrodes 64 are connected toa negative potential supply 62 to accelerate positive ions from theemitter 12, the positive ions being depicted by the path 24. A focusplate 66 and an object slit 68 of conventional design are also employedto ensure appropriate direction of the ion beam along its path 24 to theelectric sector 28. This electric sector has terminator plates 70 ateither end which are of conventional design. The sector plates 44themselves, in a typical case, may be constructed such that the innerplate is on a 7.54 centimeter radius and the outer plate is on a 17.02centimeter radius. At the point where the undeflected ion beam 40 wouldintercept the outer plate 44, an orifice or hole 38 is formed in theouter sector plate and a wire grid 72 is placed over this opening tomaintain the uniformity of the electric field within the electric sector28. These wire grids, in a typical example, may be one mil platinum wirewith a 32 mil on center spacing. The wires making up the grid areattached and electrically connected to the outer sector plate 44.

While this system has been described with reference to placing theorifice within the electric sector it may also be appropriately placedin other systems. For example, certain mass spectrometer designs existwherein the magnetic and electric sectors are transposed placing themagnetic sector first or there may only be a magnetic sector. In eithercase, a means can be provided to cause the magnetic field to be set tozero thus allowing the ion beam to pass undeflected into an electronmultiplier beam monitor as herein described. The means of setting themagnetic field to a zero level can be through the use of the well-knownHall-effect detector coupled to a feed-back circuit of conventionaldesign that would cause the magnetic power supply to be set at such alevel that achieves a zero magnetic field. A hole similar to that formedin the electric sector is formed in the magnetic sector. In thisinstance, no grid is necessary to maintain the uniformity of themagnetic field.

There has thus been described a relatively simple system whereby theundeflected ion beam is monitored to ascertain the presence of ions andat that time the system is switched on to perform a mass analysis. Thispermits, particularly in a field desorption ion source, a variation ofthe parameters within the ion source such as emitter temperature andfield strength in order to determine the particular field desorptioncharacteristics of a sample.

I claim: .[.1. In an ion beam analyzer having an ion source forgenerating ions of a sample to be analyzed, means for extracting saidsample ions from said source, means for focusing the extracted sampleions into a beam, separation means positioned along the ion beam forselectively deflecting species of ions and detecting means for detectingthe selected species ions, the improvement comprising: magnetic sectoruntil said electric sector has stabilized..].
 4. The analyzer set forthin claim .[.1.]. .Iadd.12 .Iaddend.wherein said separation meansincludes an electric sector followed by a magnetic sector, and said.[.enabling means.]. .Iadd.control .Iaddend.delays the scanning of saidmagnetic sector until said electric sector has stabilized.
 5. Theanalyzer set forth in claim .[.1.]. .Iadd.12 .Iaddend.which includesmeans responsive to said sensing means for varying .[.acharactertistic.]. .Iadd.an ion emission controlling feature .Iaddend.ofsaid sample ion source until ions are sensed.
 6. The analyzer set forthin claim 5 which includes delay means responsive to said sensing meansfor further varying said .[.characteristic.]. .Iadd.feature.Iaddend.after a predetermined period of time.
 7. The analyzer set forthin claim 5 wherein said ion source is a field desorption emitter andsaid .[.characteristic.]. .Iadd.feature .Iaddend.is emitter temperature.8. The analyzer set forth in claim .[.1.]. .Iadd.12 .Iaddend.whereinsaid separation means includes an electric sector followed by a magneticsector, said electric sector defining a hole in the path of saidundeflected ion beam, and said sensing means is located contiguous saidhole outisde said electric sector. .[.9. A method of ascertaining thefield desorption characteristics that produce ions from a sample in afield desorption ion source of an ion beam analyzer having ionseparation means comprising the steps of:energizing said ion source,disabling at least a portion of the separation means to preventdeflection of sample ions from said ion source, varying the fielddesorption characteristics of said source, and detecting saidundeflected sample ions to ascertain those field desorptioncharacteristics of said source that produce ions..].
 10. A methodaccording to claim .[.9.]. .Iadd.13 .Iaddend.wherein the additional stepof recording the field desorption .[.characteristics.]. .Iadd.feature.Iaddend.at which said sample ions are detected.
 11. A method accordingto claim .[.9.]. .Iadd.13 .Iaddend.wherein the field desorption.[.characteristic.]. .Iadd.feature .Iaddend.varied is sample temperatureor field strength to which the sample is subjected. .Iadd.
 12. An ionbeam analyzer having an ion source for generating ions of a sample to beanalyzed, comprising, in combination,means for extracting said sampleions from said source, means for focusing the extracted sample ions intoa beam, separation means capable of being energized or de-energized toselectively deflect or not to deflect species of extracted ions in saidbeam, detecting means positioned for detecting the selected deflectedspecies of ions, means to energize or de-energize said separation means,sensing means located along the ion beam for sensing said extractedsample ions prior to deflection, and control means coupled between saidsensing means and said means to energize or de-energize and responsiveto said sensed sample ions reaching a predetermined intensity level forenergizing said separation means. .Iaddend..Iadd.
 13. A method ofascertaining the features of a field desorption ion source that produceions from a sample using an ion beam analyzer having ion separationmeans comprising the steps of:energizing said ion source, disabling atleast a portion of the separation means to prevent deflection of sampleions from said ion source, varying at least one of said features of saidsource to produce sample ions, detecting the occurrence of saidundeflected sample ions, actuating said separation means for a selectedperiod of time upon detecting said occurrence, thereafter continuing tovary at least one of said features to produce sample ions, and repeatingsaid detecting and actuating steps. .Iaddend.